The principal function of any antenna is to couple an electromagnetic wave guided within the antenna structure to an electromagnetic wave propagating in free space. Many approaches exist to implement this coupling and have been intensely studied due to the vast practical applications of antennas. See, e.g., Constantine A. Balanis, Antenna Theory, 3d Ed., Wiley 2005.
In antennas based on surface scattering antennas, coupling between the guided wave and propagating wave is achieved by modulating the electromagnetic properties of a surface in electromagnetic contact with the guided wave. This controlled surface modulation may be referred to as a “modulation pattern.” The guided wave in the antenna may be referred to as a “reference wave” or “reference mode” and the desired free space propagating wave pattern may be referred to as the “radiative wave” or “radiative mode.”
Surface scattering antennas are described, for example, in U.S. Patent Application Publication No. 2012/0194399 (hereinafter “Bily I”), with improved surface scattering antennas being further described in U.S. Patent Application Publication No. 2014/0266946 (hereinafter “Bily II”). Surface scattering antennas that include a waveguide coupled to adjustable scattering elements loaded with lumped devices are described in U.S. application Ser. No. 14/506,432 (hereinafter “Chen I”), while various holographic modulation pattern approaches are described in U.S. patent application Ser. No. 14/549,928 (“hereinafter Chen II”). All of these patent applications are herein incorporated by reference in their entirety, which shall be collectively referred to hereinafter as the “MSAT applications.”
Surface scattering antennas comprise arrays of discrete radiating elements with the element spacing being typically less than about a quarter wavelength at the antenna operating frequency. Radiation from each element can be discretely modulated such that their collective effect approximates a desired modulation pattern.
Modulation has typically been accomplished in surface scattering antennas by tuning the resonant frequency of the individual radiating elements, which increases or decreases the energy coupled from the reference wave into the radiative wave. This approach typically yields a narrowband antenna, as the deeply subwavelength radiating elements are typically high-Q radiators that radiate efficiently by virtue of their bandwidth constraint.
Increased bandwidth may be desirable in applications such as broadband communications. Therefore, techniques to increase the bandwidth of a surface scattering antenna are of practical interest.